Convert Wordpress Single Install Multisite

I have single install wordpress with 10 authors.

I'm planning to convert to Multi-site, so I can create multiple subdomains and manage from single login, instead of installing several blogs.

I read tutorials about installing (and converting) from single install to multisite.

I want to know the impact of existing blog posts and its SERP rankings.

Will there be change in URL structure after do fresh install and import existing posts to new DB?

Current Blog - http://www.example.com
Multisite - http://www.example.com, sub1.example.com, etc.

Existing posts from http://www.example.com will be under http://www.example.com (multisite).

Since there has to domain mapping, I'm not sure if current SEO/SERP will be affected.